It has been a long-standing practice in the floral industry to pre-cool cut flowers for shipment in order to prolong their freshness. Pre-cooling has customarily been done by packing the cut flowers into large corrugated trays and then storing them in refrigerated warehouses until the heat was dissipated out of the flowers. This retarded the natural deterioration that takes place after the flowers are cut. A recent development has been to force-cool the cut flowers by utilizing a vacuum to suck out the warm moist air from the flowers and draw into the shipping container the low temperature ambient air in a refrigerated warehouse. It has been found that this process retards deterioration of the cut flowers for an even longer period than the afore-described conventional cooling process. The container disclosed in copending application Serial No. 906,190 was developed for use in the vacuum cooling process and incorporates a closure means associated with the container cover that may readily be opened for vacuum cooling and then quickly manually closed to maintain the interior of the shipping container in a low temperature condition. The present invention is directed to a shipping container featuring an improved closure. In the container of application Ser. No. 906,190 the closure communicated with apertures formed in the container tray. This arrangement has led to some of the cold air in the container migrating between the side walls of the tray and the cover even with the closure shut, thus reducing the container's effectiveness.